1) Field of the Invention
The present invention relates to an observation optical system and a photographing optical system. To be specific, it relates to an optical system used in an image display apparatus or a photographing apparatus, such as a camera, that can be held on the head or face of a user and that can be attached to a portable telephone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a large image. Also, in accordance with recent popularization of portable telephone and portable intelligent terminal, requirements for large view of graphics or text data on these apparatuses have grown. Conventional art has proposed to convert a displayed image on a display surface of an image display device into a virtual image to be viewed by an observer using an optical system constructed of three transmitting or reflecting surfaces each of which assumes rotational asymmetry, as disclosed by Japanese Patent Application Preliminary Publication (KOKAI) Nos. Hei 8-234137, Hei 9-197336 and Hei 9-197337. Also, U.S. Pat. No. 5,959,781 proposes to introduce a displayed image on a display surface of an image display device into a concave mirror via a total-reflection surface or a beam splitter, so as to provide an enlarged virtual image to be viewed by an observer.
However, according to KOKAI Nos. Hei 8-234137, Hei 9-197336 and Hei 9-197337, since the prism is made of a single medium and thus compensating means for compensating the chromatic aberration of magnification is not provided, insufficient compensation of the chromatic aberration of magnification makes it difficult to achieve high resolution. Also, according to KOKAI Nos. Hei 9-197336 and Hei 9-197337, the prism is rendered large in the longitudinal direction (Y direction). According to U.S. Pat. No. 5,959,781, the apparatus is rendered expensive and assembly is complicated because of the large number of parts, as well as a prism medium having extremely poor birefringence property such as glass is employed to result in heavy weight of the apparatus, because a polarization beam splitter is employed as the beam splitter to utilize polarization characteristics, etc. Also, if plastic materials are used for the optical system to solve the problem of heavy weight, the beam splitter has to be a half mirror or the like because a polarization beam splitter cannot be employed. In this case, however, light from an electronic image is attenuated at the half mirror before it reaches the eye of the observer, and thus the visible image becomes dark.
An object of the present invention is to provide an optical system specifically used in an observation optical system or a photographing optical system, which is compact and lightweight enough to be incorporated into a portable telephone, a portable intelligent terminal, or a virtual image observation system, and which provides bright images with high resolution.
An optical system according to the basic configuration of the present invention comprises, between a pupil surface and an image surface, a first prism and a second prism. The first prism comprises a 1st-1st surface which is disposed on the pupil surface side and which has both of the action of transmitting first bundles of rays (defined as bundles of rays travelling along a path between the pupil surface and the image surface via the optical system) and the action of reflecting the first bundles of rays, a 1st-2nd surface which is disposed on the opposite side of the pupil surface with respect to the 1st-1st surface and which has both of the action of transmitting the first bundles of rays and the action of reflecting the first bundles of rays, and a 1st-3rd surface which is disposed on the image surface side and which transmits the first bundles of rays. The second prism comprises a 2nd-1st surface which is disposed adjacent to the 1st-2nd surface at an air space away and which transmits the first bundles of rays at least twice and a 2nd-2nd surface which is disposed on the opposite side of the pupil surface with respect to the 2nd-1st surface and which reflects the first bundles of rays. The 1st-1st surface is constructed of a prism face of the first prism to which a holographic element is applied. The holographic element is constructed and arranged to transmit the first bundles of rays that are incident thereon at a first incident angle and to reflect by diffraction the first bundles of rays that are incident thereon at a second incident angle, to thereby have both of the action of reflecting the first bundles of rays and the action of transmitting the first bundles of rays. The 1st-2nd surface of the first prism is constructed and arranged such that a part of effective rays are incident thereon at angles greater than the critical angle so as to be totally reflected therefrom and a remaining part of the effective rays are incident thereon at angles equal to or smaller than the critical angle so as to be transmitted therethrough, to thereby have both of the action of reflecting the first bundles of rays and the action of transmitting the first bundles of rays.
According to the present invention in general, at least one surface of the first or second prism, specifically a surface having reflecting action, is rotationally asymmetric.
Also, a rotationally asymmetric surface used in the optical system of the present invention may be constructed of an anamorphic surface, a toric surface, a free curved surface that defines only one plane of symmetry. The free curved surface that defines only one plane of symmetry is particularly preferred.
Also, in the optical system according to the present invention, the prism face of the first prism is shaped as a plane base surface and the holographic element is applied to the plane base surface, to form the 1st-1st surface.
Alternatively, the prism face of the first prism is shaped as a curved base surface and the holographic element is applied to the curved base surface, to form the 1st-1st surface.
In this case, the curved base surface may be shaped as any one of a cylindrical surface, a spherical surface, an aspherical surface, an anamorphic surface, a toric surface, a surface that defines only one plane of symmetry, and a plane-symmetric free curved surface.
Also, according to the present invention, the holographic element applied to the 1st-1st surface is constructed of a volume hologram, which preferably has a rotationally asymmetric power.
Also, according to the present invention, the 1st-2nd surface of the first prism and the 2nd-1st surface of the second prism are congruently shaped so that a path of rays can be formed such that chromatic aberration of the first bundles of rays passing through the air space between the 1st-2nd surface and the 2nd-1st surface is prevented from being degraded.
In this case, the 1st-2nd surface of the first prism and the 2nd-1st surface of the second prism may be shaped as plane surfaces or alternatively, as curved surfaces.
In this case, the following Condition (1) is satisfied:
1xc3x9710xe2x88x923 (mm) less than M less than 3 (mm)xe2x80x83xe2x80x83(1)
where M is the air space between the 1st-2nd surface of the first prism and the 2nd-1st surface of the second prism.
It is much preferred that the following Condition (2) is satisfied:
1xc3x9710xe2x88x922 (mm) less than M less than 2 (mm)xe2x80x83xe2x80x83(2)
It is still much preferred that the following Condition (3) is satisfied:
7xc3x9710xe2x88x921 (mm) less than M less than 1.5 (mm)xe2x80x83xe2x80x83(3)
Also, according to the present invention, a surface having both of the refracting action and the reflecting action, such as the 1st-2nd surface, is preferably formed as a rotationally asymmetric surface. However the surface may be formed as a rotationally symmetric surface such as a spherical surface and an aspherical surface.
Also, according to the present invention, at least one of the 1st-2nd surface of the first prism and the 2nd-1st surface of the second prism is provided with an anti-reflection coating so as to prevent generation of a ghost.
The optical system having the above-described basic configuration and the additional features is applicable to an observation optical system. That is, an observation optical system according to the present invention comprises the above-described optical system and an image display element disposed on the image surface to display thereon an image to be viewed by an observer. An exit pupil is formed on the pupil surface so that an eye of the observer is positioned thereat. The observation optical system is constructed and arranged so that the first bundles of rays emergent from the image display element enter the first prism as being transmitted through the 1st-3rd surface, are then totally reflected from the 1st-2nd surface, are then reflected from the 1st-1st surface by diffraction, then exit from the first prism as being transmitted through the 1st-2nd surface, then enter the second prism as being transmitted through the 2nd-1st surface, are then reflected from the 2nd-2nd surface, then exit from the second prism as being transmitted through the 2nd-1st surface, then re-enter the first prism as being transmitted through the 1st-2nd surface, and then exit from the first prism as being transmitted through the 1st-1st surface, to be introduced to the exit pupil.
Therefore, for example, in a virtual image observation apparatus which forms an exit pupil for observation of an electronic image displayed on an image display element and which has a positive refractive power in its entirety, the observation optical system according to the present invention is constructed so that the first prism includes a rotationally asymmetric refracting surface, a rotationally symmetric or rotationally asymmetric surface having actions of total reflection and transmission, a surface which includes a cylindrical base surface or a plane base surface and a volume hologram element arranged thereon and which is disposed on the exit pupil side and that the second prism includes a rotationally symmetric or rotationally asymmetric entrance surface and a rotationally symmetric or rotationally asymmetric reflecting surface.
Also, in the observation optical system according to the present invention, the image display element is arranged such that the following condition (4) is satisfied:
40(xc2x0) less than |xcex8| less than 100(xc2x0)xe2x80x83xe2x80x83(4)
where xcex8 is an angle formed between a perpendicular to a display surface of the image display element and a visual axis, which is defined to be an axial chief ray as it passes through the exit pupil, the axial chief ray being defined to be a ray travelling between the center of the pupil surface and the center of the image surface.
It is much preferred that the following condition (5) is satisfied:
xe2x80x8360(xc2x0) less than |xcex8| less than 85(xc2x0)xe2x80x83xe2x80x83(5)
It is still much preferred that the following condition (6) is satisfied:
65(xc2x0) less than |xcex8| less than 73(xc2x0)xe2x80x83xe2x80x83(6)
Also, in the observation optical system of the present invention, the 2nd-2nd surface of the second prism maybe constructed and arranged such that a part of effective rays are incident thereon at angles greater than the critical angle so as to be totally reflected therefrom and that a remaining part of the effective rays are incident thereon at angles equal to or smaller than the critical angle so as to be transmitted therethrough.
In this case, the observation optical system may further include an optical member which transmits light and which is disposed on a side of the 2nd-2nd surface. This optical member is made of a transparent material such as glass or plastic.
Also, according to the present invention, the observation optical system may further include an optical member, such as a prism, a plane parallel glass plate and a positive or negative lens, disposed between the 1st-3rd surface of the first prism and the image display element.
Also, according to the present invention, the observation optical system may further include an optical member, such as a prism, a plane parallel glass plate and a positive or negative lens, disposed between the 1st-1st surface of the first prism and the exit pupil.
Also, in the observation optical system according to the present invention, the first prism may further include a reflecting surface arranged in a path from the 1st-3rd surface to the 1st-2nd surface so as to direct the first bundles of rays to the 1st-2nd surface by reflection.
Also, in the observation optical system according to the present invention, the second prism may further include a reflecting surface arranged in a path from the 2nd-1st surface to the 2nd-2nd surface so as to direct the first bundles of rays to the 2nd-2nd surface by reflection.
Also, in the observation optical system according to the present invention, the optical system is preferably enclosed in a dust shield member. The dust shield member preferably includes a case which accommodates the optical system and a transparent cover which transmits light traveling in the path between the first prism the exit pupil. Glass or plastic may be employed as the mazztterial of the transparent cover.
The observation optical system having the above-described basic configuration and the additional features is applicable to a head-mount image display apparatus. That is, a head-mount image display apparatus according to the present invention includes a main frame in which the above-described observation optical system is arranged as an eyepiece optical system, a support member which holds the main frame to the head of the observer so that the exit pupil of the observation optical system coincides with an eye of the observer, and a speaker member to provide a voice for an ear of the observer.
Also, the application can be modified so that a head-mount image display apparatus includes a main frame in which a pair of observation optical systems each being configured as described above are arranged, a support member which holds the main frame to the head of the observer so that the exit pupils of the observation optical systems coincide with a left eye and a right eye of the observer, respectively, and a pair of speaker members which provide voices for a left ear and a right ear of the observer, respectively.
Also, in the head mount display apparatus according to the present invention, the speaker member may be constructed of an earphone.
The optical system having the above-described basic configuration and the additional features is applicable not only to the observation optical system but also to a photographing optical system. A photographing optical system according to the present invention comprises the optical system having the above-described features, an image pickup element disposed on the image surface for photographing an image of an object, and an aperture stop disposed on the pupil surface for regulating brightness of the bundles of rays emergent from the object. The photographing optical system is constructed and arranged so that the light from the object passing through the aperture stop enters the first prism as being transmitted through the 1st-1st surface, once exits from the first prism as being transmitted through the 1st-2nd surface, then enters the second prism as being transmitted through the 2nd-1st surface, is then reflected from the 2nd-2nd surface, then exits from the second prism as being transmitted through the 2nd-1st surface, then re-enters the first prism as being transmitted through the 1st-2nd surface, is then reflected by diffraction from the 1st-1st surface, is then totally reflected from the 1st-2nd surface, and then exits from the first prism as being transmitted through the 1st-3rd surface, to be introduced to the image pickup element.
In other words, if the image display element and the exit pupil of the above-described observation optical system are replaced by the image pickup element and the aperture stop, respectively, the photographing optical system is constructed.
It is preferred that the photographing optical system also is provided with the above-described additional features of the observation optical system, such as the numerical conditions (4), (5), (6).
Furthermore, in the photographing optical system according to the present invention, liquid is employed as a medium of the second prism and a reflecting surface included in the second prism is formed as a deformable mirror, so that the second prism acts as a focusing system.